The treatment of water in water-conveying plants, such as steam generating plants, heating systems, cooling water circulatory systems and water supply systems, for protection against the corrosive action of water which is principally directed against non-noble materials, for example, steel, brass, aluminum, zinc or galvanized steel, has long found technical application. In this respect the use of compounds that contain phosphorus, as, for example, phosphonic acids or inorganic phosphates, where necessary in combination with zinc salts, has proved particularly effective.
Such combinations are technically quite effective. One such effective combination is described in U.S. Pat. No. 3,723,333 and its reissue U.S. Pat. No. Re. 28,553. This combination is that of a diphosphonic acid, an N-methylenephosphonic acid and certain orthophosphates or zinc salts or silicates or nitrates.
The use of such combinations, however, in recent times is becoming more and more restricted by ecological and legislative demands for these products to be largely or completely free of compounds containing phosphorus. From the technical viewpoint, combinations of this kind containing phosphorus have the further disadvantage that they frequently lead to eutrophication of the cooling system due to intensified biological growth and therefore require the additional use of microbicides.
The use of such combinations containing phosphorus can further lead, when applied to water of great hardness, to the formation of apatite sediments or sediments similar to apatite, that lead to operating troubles and can only be removed with difficulty. The application of the said combinations at higher pH values (pH&gt;8.0) generally leads to a clogging of the system by the precipitation of zinc hydroxide.